Effect of 4d Transition Metal Atom Rh Doping on Thermoelectric Power, Magnetic Susceptibility, Thermal Expansion and X-Ray Photoemission Spectra in the Charge-Transfer Type Nonmetallic State of NiS

We report the following two remarks of the Rh impurity effect on the charge-transfer (CT) type nonmetallic state (below the nonmetal-metal transition temperature: T_t) of 3d transition metai compound NiS. (1) Although the nearest neighbourmg M-M (Ni or Rh) and S-S distances increase with increasing x, dT_t/dx<0. The Rh substitution gives a broad peak with the Fermi edge. A positive sign of T-linear contribution of the thermoelectric power turns to negative one at the critical concentration (x_c) in Ni_1−xRh_xS. Below T_t, a Curie constant is observed and increases with increasing x. It is suggested that an impurity band introduced by the Rh atoms leads a change of the electronic state of Ni atoms and the suppression of the CT gap. (2) An empirical relation (dT_t/d | C_N| >0) is found on Ni_1−xRh_xS, Ni_1−dS and NiS_1−ySe_y, where C_N is the phonon-drag coefficient. The slope dT_t/d | C_N| becomes smaller in Ni_1−xM_xS with the early 3d transition metal impurities (M: Ti, V and Cr).